1. Field of the Invention
The present invention relates to a sound reproducing system to reproduce sound by performing a virtual sound source process on audio signals of part of a plurality of channels. The present invention also relates to a sound reproducing method in the sound reproducing system.
2. Description of the Related Art
A video and sound reproducing system called a “home theater system” has been becoming widespread. In the video and sound reproducing system, video is reproduced from a DVD (digital versatile disc) or the like by displaying it on a display having a relatively large screen, while sound is reproduced in a multi-channel surround sound method, recently in a 5.1 channel method. Accordingly, the video and sound can be powerfully reproduced.
In a sound reproducing system of the 5.1 channel method, four types of speakers are used: one located in front of a listener (hereinafter referred to as “front”); one located at the center in front of the listener (hereinafter referred to as “center”); one located at the rear of the listener (hereinafter referred to as “rear”); and one dedicated to low frequencies. A sub-woofer, which is a speaker dedicated to low frequencies, originally handles a frequency band of 100 Hz or less in monaural. The other speakers handle 100 Hz to 20 KHz.
FIG. 13 illustrates placement of speakers in a conventional sound reproducing system of the 5.1 channel method. That is, as illustrated in FIG. 13, a speaker 10FL for a front-left channel is placed on the left in front of a listener 4, a speaker 10FR for a front-right channel is placed on the right in front of the listener 4, and a speaker 10C for a center channel is placed at the center in front of the listener 4.
Also, a speaker 10RL for a rear-left channel is placed on the left at the rear of the listener 4 and a speaker 10RR for a rear-right channel is placed on the right at the rear of the listener 4. Furthermore, a sub-woofer speaker 10SW for a LFE (low frequency effect) channel (dedicated to low frequencies) is placed at an appropriate position.
These six speakers 10FL, 10FR, 10C, 10RL, 10RR, and 10SW are attached to speaker boxes, respectively, and are positioned at respective positions. Typically, the six speakers on the front and rear sides are placed such that a distance ds from the listener 4 is about two meters, for example.
In the conventional sound reproducing system, speaker boxes of about 15 litters used in the speakers for front-left and front-right channels have been replaced by small boxes of around one litter, and such speakers are called satellite speakers. Of course, those speakers do not output sound of low frequencies, and thus a speaker dedicated to low frequencies called a sub-woofer is added to the system. As in this case, when the speakers other than the sub-woofer are small boxes, the crossover frequency of audio signals supplied to the sub-woofer 10SW is 150 Hz, slightly higher than the above-mentioned 100 Hz, but this frequency is still low.
When audio signals of the 5.1 channel from a DVD are reproduced in the speaker system of the above-described placement, sufficient bass sound can be reproduced as a matter of course. Furthermore, a channel dedicated to low frequencies is provided on a reproducing side. Thus, when the sound source is a movie, deep bass sound that could not have been conventionally obtained echoes throughout a room, and powerful and realistic sensation can be obtained.
However, in a relatively small room, it may be impossible to make room for placing the above-described six speakers to reproduce multi-channel surround sound. Also, the necessity for six speakers and amplifiers leads to high cost disadvantageously.
Noise caused by sound leakage is also a problem. For example, volume of about 90 dB or more is required to reproduce powerful sound in video and sound appreciation of a DVD in a typical 5.1-channel speaker configuration. That is, if a listener wants to obtain a favorable effect of the multi-channel surround, he/she needs to consider a noise problem.
An example of a method for solving the above-described problems is a virtual sound source process (virtual sound image localization process) described in Patent Documents 1 and 2 (Japanese Unexamined Patent Application Publications Nos. 9-327099 and 10-224900).
In this process, audio signals to be supplied to two actually-placed speakers are generated. The audio signals are processed based on a transfer function (HRTF: head-related transfer function) to both ears of a listener when speakers are placed in positions where a virtual sound image is localized.
By using this virtual sound source process, the above-described multi-channel surround sound of the 5.1 channel can be reproduced with only two speakers. Accordingly, the space can be saved and the cost can be reduced.